1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly to a color cathode ray tube which realizes an optimum focusing on the whole screen by decreasing the non-uniformity of beam spot shape attributed to a deflection quantity.
2. Description of the Related Art
In general, a color cathode ray tube is constituted of a vacuum envelope which is comprised of a panel portion forming a display part (a phosphor screen or a screen), a narrow-diameter neck portion and a funnel-like funnel portion which connects the panel portion and the neck portion, and a deflection yoke is mounted on an outer portion of a connecting area between the panel portion and the neck portion. On an inner surface of the panel portion which constitutes the screen, a phosphor surface (a phosphor screen) which is formed by applying phosphors of three colors consisting of red, green and blue usually is formed. In a so-called shadow mask type, a shadow mask which constitutes a color selection electrode is arranged close to the phosphor screen.
Further, in the inside of the neck portion, an electron gun which emits three electron beams corresponding to the above-mentioned phosphors of three colors in parallel in the horizontal direction is usually housed. By allowing three electron beams emitted from this electron gun to pass through beam apertures formed in the shadow mask, three electron beams are made to impinge on the phosphors of three colors constituting the phosphor screen so that a color image is reproduced.
The electron gun housed inside the above-mentioned neck portion includes an electron beam generating part (a triode part) which sequentially arranges a cathode, a control electrode which is arranged close to the cathode and has three respective electron beam apertures consisting of a center electron beam aperture and side electron beam apertures in an in-line arrangement in a state that these beam apertures respectively face the cathode and an accelerating electrode, and a focusing electrode and an anode for focusing and accelerating electron beams generated by the electron beam generating part.
FIG. 10 is a schematic cross-sectional view for explaining one constitutional example of an electron gun in a conventional color cathode ray tube. In the drawing, an electron beam generating part (a triode part) is constituted of a cathode K which is heated by a heater H, a first electrode G1 which constitutes a control electrode and a second electrode G2 which constitutes an accelerating electrode. Electrons which are generated by the cathode K are formed into electron beams B after passing through the triode part. The electron beams B pass through a pre-focusing lens which is formed by a focusing electrode constituted of a third electrode G3, a fourth electrode G4 and a fifth electrode G5, and is emitted in the direction toward a phosphor screen from a main focusing lens which is formed between the fifth electrode G5 and a sixth electrode G6 which constitutes an anode. Here, SC indicates a shielding cup. FIG. 10 shows a cross section in the vertical direction (longitudinal direction) V with respect to the center beam among three electron beams emitted in parallel in the horizontal direction (lateral direction) H. The same goes for respective drawings described hereinafter.
Reference symbol G1-H indicates an electron beam aperture of the first electrode G1, reference symbol G2-H indicates an electron beam aperture of the second electrode G2, reference symbol G3-BH indicates electron beam apertures formed in the second electrode G2 side of the third electrode G3, that is, electron beam apertures formed in a bottom of the third electrode, reference symbol G3-TH indicates electron beam apertures formed in the fourth electrode G4 side of the third electrode G3, that is, electron beam apertures formed in a top of the third electrode, reference symbol G4-H indicates electron beam apertures formed in the fourth electrode G4, reference symbol G5-BH indicates electron beam apertures formed in the fourth electrode G4 side of the fifth electrode G5, that is, electron beam apertures formed in a bottom of the fifth electrode, reference symbol G5-TH indicates electron beam apertures formed in the sixth electrode G6 side of the fifth electrode G5, that is, electron beam apertures formed in a top of the fifth electrode, reference symbol G6-BH indicates electron beam apertures formed in the fifth electrode G5 side of the sixth electrode G6, that is, electron beam apertures formed in a bottom of the sixth electrode, and reference symbol SC-H indicates electron beam apertures formed in the shielding cup SC. Here, the electron gun shown in FIG. 10 constitutes merely one example and various modifications are considered as the electrode constitution ranging from the focusing electrode to the anode.
FIG. 11A and FIG. 11B are schematic plan views for explaining the constitutions of the second electrode and the third electrode shown in FIG. 10. That is, FIG. 11A is a plan view of the second electrode G2 as viewed from the first electrode G1 side, that is, the bottom side of the second electrode, while FIG. 11B is a plan view of the third electrode G3 as viewed from the second electrode G2 side, that is, the bottom side of the third electrode. In a bottom G2-B of the second electrode G2, laterally-elongated concave SL-H having a long axis in the horizontal direction are formed such that the laterally-elongated concave SL-H surround the electron beam apertures G2-H. On the other hand, in a bottom G3-B of the third electrode G3, longitudinally elongated electron beam apertures G3-BH having a long axis in the vertical direction V are formed.
Further, in an electron gun described in the U.S. Pat. No. 5,600,201 specification, electron beam apertures formed in a first electrode G1 are laterally elongated having a long axis in the horizontal direction, and a second electrode G2 has laterally-elongated concave having a long axis in the horizontal direction at the first electrode G1 side while surrounding the electron beam apertures, and key-hole-shaped apertures each of which is formed by providing elongated open apertures extending in the upward and downward directions to a circular opening are formed in the bottom of the third electrode G3. Here, the combined structure in which laterally-elongated concave which surround the electron beam apertures are formed at the bottom or the top of the second electrode G2, and longitudinally elongated electron beam apertures or longitudinally elongated concave which surround the electron beam apertures are formed in the bottom of the third electrode G3 is also known.